The present invention relates generally to an apparatus and method for reducing head stiction in a disc drive, and more particularly, to reducing head stiction caused by outgassing from components in the disc drive.
Hard disc drive storage devices have been used in computer systems for many years. In a disc drive, a disc is rotated by a spindle motor which supports the disc. The rotation of the disc creates a thin film of air over the surface of the disc. An actuator positioned adjacent to the disc supports a transducer which reads and writes data to and from concentric tracks of the disc. The transducer, which is incorporated into or supported on a slider, is supported by the thin film of air as it travels over the disc surface. Data storage on the disc is partially a function of the distance between the transducer and the surface of the disc when the disc drive is operating. To increase storage capacity, the transducer is typically positioned close to the surface of the disc (i.e., fly height) such that the dispersion of the interaction between the transducer and the disc surface for each data transition point is minimized.
In order to lower flying heights, the disc surface must be made as smooth as possible. However, the disc surface cannot be made perfectly smooth because of the need to avoid "stiction". For a head-disc assembly ("HDA"), stiction may be defined as the lateral force required to overcome the separation between the slider and the surface of the disc caused by contamination in the HDA.
One known problem in disc drives is that drive heads which support the transducers come into contact with and "stick" to the disc surface when the disc stops rotating. Typically, discs rotate between 5,400 and 10,000 revolutions per minute during normal operation. Disc lubricants may be used to prevent the head from sticking to the disc surface. These lubricants are not sufficient to reduce the stiction caused by components of the disc drive which outgas particulates, especially when the components are subject to heat. Since the HDA is generally located in a relatively sealed environment, outgassed components are subject to a partial equilibrium condition at a given pressure, altitude, and temperature. Thus, the concentration of outgassed material is higher in this environment because these gases are not diluted by external air. During the operation of the disc drive, the atmospheric pressure between the head and the disc increases which also increases the amount of outgassed particulates. The outgassed material has a tendency to condense on cold surfaces, for example, the disc surface. The head at rest surface sticks to the surface of the disc, thereby causing damage to the disc surface because of the affected stiction. Further, the head sticking interferes with the proper flying of the head that can result in either head crashes or improper recording during head operation. Stiction also occurs if the head and disc surfaces are too smooth. In this case, molecules tend to migrate between the two surfaces and eventually join the two surfaces under sufficient temperature and pressure.
When liquid lubricants are introduced to reduce wear during the context start/stop (CSS), stiction is even higher than when no liquid is present at the disc surface. Thus, it is desirable that the head-disc and interface have both low wear from the head landing on, or periodically contacting the surface of the disc, and low stiction, so that the rotation of the disc can be restarted after the disc has been stopped without damage.
Recently, the magnetic storage industry has continued the trend towards higher storage capacity by lowering the flight height of the read/write head over the disc surface. Factors that affect stiction of a HDA include head load, slider size, surface roughness, geometrical conformity between slider and disc. Other factors include the physical and chemical properties of lubricants and lubricant film thickness, physical and chemical properties of slider and disc overcoat materials, and environmental factors such as temperature and humidity.
Several methods are known for reducing head stiction. One such method is to pulse the spindle motor at the resonant frequency of the disc to provide a force which may "unstick" the head. Similarly, the motor may also be pulsed at the resonant frequency of the disc. One disadvantage to these methods is that the creation of the resulting force may cause damage to the head, transducer, or other suspensions in the disc drive.
In another method, a cam or ramp may be used to lift the head off the disc when the disc drive is powered down. In another method, the contamination levels on components may be tightly controlled or materials with low outgassing properties may be used during the manufacture of the disc drive. In another method, recirculating chemical filters are used to filter out contaminants, including outgassed materials. In other methods, components such as the actuator coil are enclosed in order to reduce the stiction caused by the outgassing.
Therefore, a need exists for a method and apparatus to reduce contamination levels in a disc drive which is efficient and low cost.